+ Post New Thread
Results 1 to 9 of 9

28th March 2011, 18:21 #1
 Join Date
 Apr 2007
 Posts
 201
 Helped
 6 / 6
 Points
 2,684
 Level
 12
AC Analysis of single opamp differential amplifier Pspice simulation
Hi,
Im trying to do a ac analysis of a opamp differential amplifier the circuit is shown below.
The input is fed with a common mode ac signal 1 vpp.
The differential stage is given a 6V offset too..
So for the common mode signal the output ideally will be 6V dc signal only.But when i do the AC analysis im getting something like this..
Can somebody tell me why

28th March 2011, 22:48 #2
 Join Date
 Mar 2011
 Location
 Australia
 Posts
 40
 Helped
 9 / 9
 Points
 501
 Level
 4
Re: AC Analysis of single opamp differential amplifier Pspice simulation
The transient in the waveform is around BW cutoff of the circuit. Can you elaborate more, what exactly are you showing? Is is written there "N differential..?"

28th March 2011, 22:48

28th March 2011, 23:04 #3
 Join Date
 Jan 2008
 Location
 Bochum, Germany
 Posts
 39,450
 Helped
 12060 / 12060
 Points
 228,812
 Level
 100
Re: AC Analysis of single opamp differential amplifier Pspice simulation
Your circuit measures common mode rejection ratio, as expectable for any real OP, it's below infinity...
The measurement correspondends almost exactly to the datasheet's CMRR specification of about 60 dB at 1 MHz.
1 members found this post helpful.

28th March 2011, 23:04

29th March 2011, 01:34 #4
 Join Date
 Apr 2007
 Posts
 201
 Helped
 6 / 6
 Points
 2,684
 Level
 12
Re: AC Analysis of single opamp differential amplifier Pspice simulation
Your circuit measures common mode rejection ratio, as expectable for any real OP, it's below infinity...
The measurement correspondends almost exactly to the datasheet's CMRR specification of about 60 dB at 1 MHz.
what im asking is that,,,,Isn't the output voltage should rise frm the offsetted 6v dc rather than from 0v dc,after 100kHz?
For zero differential input i ve offsetted the opamp at 6v dc.
Thanks For any help
RegardsLast edited by themaccabee; 29th March 2011 at 05:35.

29th March 2011, 01:34

29th March 2011, 09:51 #5
 Join Date
 Apr 2007
 Posts
 201
 Helped
 6 / 6
 Points
 2,684
 Level
 12
Re: AC Analysis of single opamp differential amplifier Pspice simulation
or does the ac analysis will kill all the dc sources?? and show only relative values?? or is there some error in my circuit?

29th March 2011, 10:20 #6
 Join Date
 Jan 2008
 Location
 Bochum, Germany
 Posts
 39,450
 Helped
 12060 / 12060
 Points
 228,812
 Level
 100
Re: AC Analysis of single opamp differential amplifier Pspice simulation
or does the ac analysis will kill all the dc sources??
1 members found this post helpful.

29th March 2011, 10:22 #7
 Join Date
 Jul 2007
 Posts
 20
 Helped
 3 / 3
 Points
 1,326
 Level
 8
Re: AC Analysis of single opamp differential amplifier Pspice simulation
Your sim results are as expected. AC analysis doesnot kill dc sources, it will take dc points before calculating AC analysis data. You are not offsetting the opamp as you mentioned but giving common mode signal on dc 6V . Are u clear?
1 members found this post helpful.

29th March 2011, 10:22

29th March 2011, 11:10 #8
 Join Date
 Apr 2007
 Posts
 201
 Helped
 6 / 6
 Points
 2,684
 Level
 12
Re: AC Analysis of single opamp differential amplifier Pspice simulation
Your sim results are as expected. AC analysis doesnot kill dc sources, it will take dc points before calculating AC analysis data. You are not offsetting the opamp as you mentioned but giving common mode signal on dc 6V . Are u clear?
Vo=Vref+Vdiff*G
Vref=6V
G=1
So thats becoz i asked so.
I think now it is becomming clear..
Thanks..

29th March 2011, 11:38 #9
 Join Date
 Jan 2008
 Location
 Bochum, Germany
 Posts
 39,450
 Helped
 12060 / 12060
 Points
 228,812
 Level
 100
Re: AC Analysis of single opamp differential amplifier Pspice simulation
AC analysis is small signal analysis. It determines the change in output voltage generated by a small (or as the mathematicans say infinitesimal) change in input voltage. So the DC level is always ignored.
1 members found this post helpful.
+ Post New Thread
Please login